Recently, in association with the movement of global regulation of carbon dioxide emission associated with the increase in attraction to environmental concerns, the demand for low fuel consumption by automobiles is increasing. To satisfy the requirement, it is desired to reduce rolling resistance relating to tire performance. Heretofore, as a means for reducing the rolling resistance of tires, a method of optimizing tire structures has been investigated; however, at present, a technique of using a low-heat-generating rubber composition for tires has become employed as the most common method.
For obtaining such a low-heat-generating rubber composition, there is known a method of using an inorganic filler such as silica or the like.
However, in the rubber composition containing silica compounded therein, silica aggregates (owing to the hydroxyl group in the surface of silica), and therefore, for preventing the aggregation, a silane coupling agent is used.
Accordingly, for successfully solving the above-mentioned problem by incorporation of a silane coupling agent, various trials have been made for increasing the activity of the coupling function of the silane coupling agent.
For example, PTL 1 proposes a rubber composition containing, as basic components, at least (i) one diene elastomer, (ii) a white filler as a reinforcing filler and (iii) an alkoxysilane polysulfide as a coupling agent (white filler/diene elastomer) along with (iv) an enamine and (v) a guanidine derivative.
PTL 2 discloses a rubber composition containing, as basic components, at least (i) one diene elastomer, (ii) a white filler as a reinforcing filler and (iii) an alkoxysilane polysulfide as a coupling agent (white filler/diene elastomer) along with (iv) zinc dithiophosphate and (v) a guanidine derivative.
PTL 3 describes a rubber composition containing, as basic components, at least (i) a diene elastomer, (ii) an inorganic filler as a reinforcing filler and (iii) an alkoxysilane polysulfide (PSAS) as an (inorganic filler/diene elastomer) coupling agent, as combined with (iv) an aldimine (R—CH═N—R) and (v) a guanidine derivative.
Further, PTL 4 proposes a rubber composition basically containing at least (i) a diene elastomer, (ii) an inorganic filer as a reinforcing filer and (iii) an alkoxysilane polysulfide as a coupling agent, as combined with (iv) 1,2-dihydropyridine and (v) a guanidine derivative. PTL 5 proposes a technique of increasing the activity of the coupling function of a silane coupling agent in consideration of kneading conditions.
PTL 6 describes an invention, in which a silica having an average particle diameter of 10 μm or less and a specific silane coupling agent are added to a rubber composition, thereby suppressing aggregation of the silica.
PTL 7 proposes a technique, in which a silica that preferably has a n-hexadecyltrimethylammonium bromide (CTAB) adsorption specific surface area of from 60 to 250×102 m2/kg and an tea extract containing catechin are added to a rubber composition, thereby preventing large aggregates of the silica from being present in the rubber composition.
Furthermore, PTL 8 and PTL 9 describe a rubber composition that has such a dispersion state that the area ratio occupied by filler aggregates having a circle-equivalent diameter of 10 μm or more based on the total observed area in a dispersion evaluation method of observing a cut surface of a specimen by a dark field method is 2.0% or less.
However, there is a demand of a technique for further enhancing the low-heat-generation property of a rubber composition containing silica.